Ask any teenager about healthy eating habits and the first thing you will be told is the importance of a low-fat diet. Of all the dietary messages that have become gospel over the past 30 years, the low-fat-equals-low-cholesterol-equals-longer-life one is by far the most successful, writes Muiris Houston.
And, like any dogma, it seems to have had its day. The science of dietary fat, upon which a clear public-health message has been built, is being questioned by doctors and scientists alike.
The December 1st issue of the British Medical Journal has the latest research to question our beliefs. In a study of more than 14,500 people living in the county of Norfolk in England, doctors have discovered that cholesterol levels are inversely related to the frequency of eating in both men and women.
In other words, the more often you eat, the lower the concentration of cholesterol in your bloodstream. Average cholesterol levels differed by about 0.25mmol/L between people eating more than six times a day and those who ate only once or twice.
A crude message from this study might be that if you are a gorger, eating infrequently but in large amounts, you are at greater risk from heart disease; if you are a nibbler, eating little and often, you are in some way protected.
But of course this presumes we accept a link between cholesterol levels in the general population and heart disease. It also assumes a proven link between dietary-fat intake and the likelihood of developing coronary problems.
Science magazine, in an article entitled "The soft science of dietary fat", tackled this issue in some depth earlier this year.
"During the past 30 years, the concept of healthy eating in America has become synonymous with avoiding dietary fat. The food industry now spends billions of dollars yearly selling the less-fat-is-good message.
"The low-fat gospel spreads further by a kind of societal osmosis, continually reinforced by physicians, nutritionists, journalists, health organisations and consumer-advocacy groups," it said.
The main thesis put forward by the author, Gary Taubes, is that the science of dietary fat is not nearly as simple as it first appeared. While acknowledging that each step from fat to cholesterol to heart disease has been demonstrated beyond reasonable doubt, he questions whether the veracity of the chain as a whole has ever been proven.
Death rates from heart disease may be declining, but Taubes suggests that this is because doctors are treating it more successfully rather than because there is any reduction in the number of new cases of heart problems.
This connection would seem to be backed up by figures from the American Heart Association, which show that the number of medical procedures for heart disease increased from 1.2 million to 5.4 million a year between 1979 and 1996.
During the same period, the average fat intake in the US dropped from more than 40 per cent of total calories to 34 per cent, yet obesity levels went from 14 per cent to 22 per cent of the population. The number of people with diabetes also rose significantly.
This suggests - although it cannot be proven - that low-fat diets may have had the unintended consequence of causing weight gain. It also raises the possibility that, by replacing fat with carbohydrate, we have contributed, at least in part, to the obesity epidemic.
Low-fat diets do bring about an initial weight loss, which then tends to return. But there has to be a dietary trade-off. If you eat less of one thing, you are likely to eat more of another. And as fat and protein tend to coexist in the same foods, it follows that a person on a low-fat diet will eat more carbohydrate.
Carbohydrates encompass a very wide range of foods, from those with a high simple sugar content to fresh green vegetables. Guess which looks more attractive on a supermarket shelf: fresh fruit or a highly processed fast-food carbohydrate.
But in eating large amounts of fruit and vegetables - the typical Mediterranean diet - we also consume olive oil, which may also have a role to play in the low death rate from heart disease in southern Europe.
What is not highlighted about Italians and Spaniards is that their consumption of animal fat has risen. Despite this, the rate of heart disease continues to decline, which puts a large dent, to say the least, in the fat-is-bad argument.
There is one important caveat, however. For people with family histories of heart disease and other risk factors, lowering cholesterol remains an important part of preventing a heart attack.
Maurice Neligan, the doyen of cardiac surgeons in the Republic, acknowledges the importance of cholesterol-lowering drugs, especially in the secondary prevention of heart disease.
Yet when I asked him, as one who has been sceptical about the low-fat argument, for his view on the Science article, I'm sure I could detect a wry smile over the telephone line.
He confined himself to saying: "I am not surprised, but I am pleased that the topic has been revisited from a scientific and not a populist viewpoint."
So where does this leave those of us planning a cardio-protective diet? Confused, I suppose, and although the experience of the past 30 years suggests that there is no simple answer, the days of demonising fat appear to be over.
For what it is worth, here is my prescription: a varied diet, balancing fat, protein and carbohydrate; lots of exercise, which in my experience is often neglected; modest amounts of wine; and, if you're a smoker, quit!
E-mail Dr Muiris Houston, Medical Correspondent, at mhouston@irish-times.ie or leave a message at 01-6707711 ext 8511. He regrets he cannot reply to individual medical problems